
179
Engineering
information
HUBER+SUHNER Microwave assembliesAbbreviations see page 182
Engineering information
Intermodulation distortion in passive components
Intermodulation distortion in passive microwave components is
caused by internal nonlinearities. In a truly linear system, the
output is directly proportional to the input.
In a nonlinear system, the output signal is distorted by changes
in the amplitude of the input signal. Intermodulation distortion
creates new output signals from the nonlinear combinations of
two or more input signals mixed together. A nonlinear circuit
will create an infinite number of harmonics from two fundamen-
tal frequencies (f1 and f2). A particular concern for telecommu-
nication systems engineers is the intermodulation product of the
third order (such as 2f1 – f2 and 2f2 – f1), especially if f1 and
f2 are closely spaced. With certain system designs and band-
width allocations, the third order intermodulation products can
be generated at the same frequencies as the receive channels
of the system. In general, intermodulation products increase
system noise and reduce the number of available channels.
Intermodulation distortion is most pronounced in systems where
the high power transmission and low power receiver signals
are carried simultaneously in the same transmission line, such
as in the cable between the duplexer and the antenna in GSM
base stations and in certain space applications. For low power
levels, the effects of intermodulation distortion are significantly
less. HUBER+SUHNER is involved in the research of the inter-
modulation problem as a participant in the IEC TC46 WG6
passive intermodulation working group.
Coaxial cable assemblies have often been viewed as linear
components. However, pure linear components do not exist.
There are small nonlinearities in the connectors and in the ca-
ble to connector junctions. Intermodulation distortion in con-
nectors is usually caused by thin surface oxide layers at the
connector junctions or by insufficient contact pressure when
the current-carrying contact zones become separated. Sepa-
ration is usually microscopic and can be caused by either elec-
tron tunneling or microscopic arcing. The presence of ferroma-
gnetic materials in the current path may also contribute to
intermodulation distortion.
Some simple design rules can help avoid intermodulation dis-
tortions in coaxial cable assemblies:
• Use of semi-rigid cable with a seamless outer conductor in
place of flexible cable.
• Use of a solid center conductor in place of a stranded
center conductor.
• Directly attach the outer conductor to the connector body
by soldering or clamping in lieu of crimping.
• Limit the number of parts in the current path.
• Eliminate contaminants in the current path.
• Use high quality machining in the connector parts with a
smooth surface finish.
• Avoid contaminants in the plating solutions.
• Ensure adequate and uniform plating thickness.
• Avoid use of magnetic materials in the current-carrying path.
• Ensure adequate contact pressure.
• Contact surface of female contact fingers should cover as
close to 360 as possible (i. e. narrow slots or slotless).
• Use connector interfaces with radial dimensions as large as
possible (7/16 over N, N over SMA).
Space applications
Every space application is unique and requires careful consi-
deration before selecting the components to be used. A space
environment subjects components and assemblies to severe
environmental stress:
• Low earth orbit spacecraft subject solder joints, welds,
brazements and mechanical connections to continuous hot/
cold thermal cycling every 90 minutes. The manufacturing
process must be carefully controlled per NASA STD-8739
requirements to assure consistent, reliable connections and
assemblies. Solder connections must be 100 % X-rayed to
assure their integrity and reliability.
• There is no atmosphere so convection cooling does not
occur. Excess heat must be removed by radiation, which
requires the surface of the connectors to be an infrared
emitter, or by conduction which requires a secondary
heat sink.
• Certain materials "outgas" in the extreme vacuum of space
which requires the designer to select materials and compon-
ents that meet NASA requirements for Total Mass Loss (TML)
and Collected Volatile Condensable Material (CVCM) to
avoid contamination of optics and other sensitive equipment
on board the spacecraft.
• Materials must be carefully chosen so that ionising radiation
does not destroy the connector or cable dielectric or the
cable jacket.
• Multipaction failure (described in more detail herein) is a con-
cern for high power applications.
• Intermodulation distortion (described in more detail herein) is
a concern within systems where high power transmitting and
low power receiving signals need to be carried simultane-
ously in the same transmission line.
• The manufacturing environment must be carefully controlled
and the packaging materials selected to prevent dust and
particles from accumulating on the components and subse-
quently contaminating the spacecraft.
Processes and controls used by HUBER+SUHNER for procure-
ment, manufacture, assembly, soldering, X-ray, inspection and
testing have been certified by NASA for use in spacecraft
applications. HUBER+SUHNER has the design, manufacturing,
testing and applications experience and expertise to supply
your needs for passive microwave devices for use in any space
environment.